Novel articles have been discovered having a flatted surface comprising thermoplastic polymer resins and an effective amount of a novel multi-stage polyorganosiloxane/polyvinyl-based graft polymer flatting agent. Many benefits can be realized from articles containing this new polyorganosiloxane/polyvinyl-based graft polymer flatting agent. For example, the flatting agent lends improved impact strength to the articles as well as reduced gloss.
Many thermoplastic polymers when thermoformed by casting or molding, and especially injection molding, calendering and extrusion, and the like, have smooth glossy surfaces. While such surfaces are highly desirable in many applications, there are certain cases wherein they are distinctly disadvantageous. For example, glossy surfaces resist painting because the paints, inks and varnishes tend not to adhere; smooth surfaces mar and abrade, developing a hazy appearance; smooth thermoplastic surfaces tend to stick together when stacked, the so-called blocking phenomenon, and smooth surfaces reflect light. Light reflection, especially, is a problem because this can cause operators of business machines molded with glossy surfaces to become tired after prolonged usage, because stray light is reflected into their eyes.
Thermoplastics for which surface altering is especially needed are those which injection mold into high strength pieces having highly glossy surfaces. These include the engineering thermoplastics, such as aromatic polycarbonates; polyester carbonates; aromatic polysulfone carbonates; polyester resins comprising units of an aliphatic diol, a cycloaliphatic diol or a mixture of such diols and an aromatic diacid; vinyl aromatic polymers; polyphenylene ethers, mixtures of any of the foregoing and the like. Business machine housings, especially cathode ray tube housings, and picture tube protective shields, molded from such polymers have a high specular gloss at 60 degrees, according to ASTM Standard Test Method D 523, and a significant reduction in such gloss is desirable, while still maintaining all of the other valuable properties of such materials, especially strength and flame retardancy, in certain embodiments.
Previous approaches to altering the surface of thermoplastics have involved the use of additives or mechanical treatments such as calendering or sandblasting. The mechanical treatments cannot generally be used in injection molding and extrusion processes. With respect to the additive approach, this generally comprises using inorganic particulate materials, such as calcium silicates, magnesium silicates, amorphous silica gels and the like. Although these work rather well in plasticized poly(vinyl chloride) resins, they are difficult to compound into the high melting engineering thermoplastics which are used in high quality business machine housings, and the like. The difficulty is in obtaining uniformly dispersed particles on the surface of the articles so that predictable gloss reduction is obtained, while at the same time not reducing physical strength. Other additives have been proposed, selected from organic polymeric materials. Canadian Pat. No. 1,049,185, for example, describes cross-linked acrylate polymers, and states that they are useful in a wide range of thermoplastics, including polycarbonates and mixtures thereof, but then goes on to state that the cross-linked polymer should be particulate in form, having an average particle size of 1 to 30 microns. This causes the same difficulty in dispersion and non-uniform gloss-reduction as is commonly experienced when particulate inorganic additives are employed and the resulting composition is injection molded and/or extruded. A further approach suggested for use in poly(vinyl chloride) compounds used in house siding and other rigid profile extrusions, is to blend into the vinyl halide resin a small amount of an impact modifier, such as ethylene/vinyl acetate copolymer and then to add a dulling agent. In a Technical Bulletin on terpolymer resin modifiers, ELVALOY 837 and 838, the DuPont Company has suggested that if a terpolymer of ethylene, vinyl acetate and carbon monoxide is used in poly(vinyl chloride) a desirable matte finish can be obtained without dulling agents. Compositions comprising such terpolymers and a wide variety of other resins, especially poly(vinyl chloride) resins, but including polycarbonates and polyesters, are taught in Hammer, U.S. Pat. No. 3,780,140, but there is no suggestion therein that molded articles having reduced gloss can be obtained without a dulling agent.
Allen, et al., in commonly assigned, U.S. Pat. No. 4,536,542, discloses the use of a terpolymer comprising ethylene, a carbon monoxide and a third monomer as a flatting agent to achieve desired altering of the surface of the article.
There have also been many attempts in the art to provide polyorganosiloxane-based graft polymers which may be useful as impact strength modifiers for thermoplastic resins. See, for example, U.S. Pat. No. 2,891,920 (J. F. Hyde, et al.); and O. Graiver, et al., Rubber Chem. Tech., 56 (5), 918 (1983).
The major deficiencies which have prevented the widespread use of polyorganosiloxane impact modifiers in thermoplastic resins have included raw material and the thermoplastic resin. Additionally, the siloxane polymerization process requires careful control to eliminate contamination of the silicone rubber by linear or cyclic siloxane oligomers. Surface delamination in molded thermoplastic parts has been partially attributed to the presence of such oligomer contaminants in the silicone rubber.
Furthermore, polyorganosiloxane compounds generally exhibit low reactivity toward vinyl monomers during the course of subsequent graft polymerization. Several methods for improving graft efficiency have been reported. U.S. Pat. No. 3,898,300 states that a polyorganosiloxane-based graft copolymer for improving the impact strength of S/AN resin is formed by grafting S/AN comonomers in an emulsion system onto the vinylsiloxane- or allylsiloxane-containing silicone substrates. U.S. Pat. No. 4,071,577 describes a similar approach by using a mercaptosiloxane in place of vinyl-group containing siloxanes. European Patent Application No. 0,166,900 reports further improvement of polysiloxane-based graft polymers and increased S/AN impact strength by using an acryloxy-functionalized siloxane as the graft-linking agent. These graft polymers are utilized in connection with the impact modification of S/AN. British Pat. No. 1,590,549 describes the use of a polyorganosiloxane graft polymer in various plastic molding compositions. Similarly, European Patent Application No. 0,249,964 describes the use of a polyorganosiloxane graft polymer in the polycarbonate containing blends..
Mention is also made of European Patent Application Nos. 0,246,537 and 0,260,552, both of which describe the use as impact modifiers of a polyorganosiloxane polymer substrate on which are subsequently grafted first and second vinyl-based polymer stages. The second of said applications also describes soaking the first stage substrate with the second stage monomer(s) to cause an "entangling" thereof with the silicone prior to subsequently polymerizing the second stage.
The polyorganosiloxane-based rubber utilized in the foregoing references tends to have poor mechanical properties, poor appearance properties, a tendency to cause delamination in final molded products and generally offers poor impact strength performance in typical thermoplastic applications.
Each of these disadvantages can be overcome by improving the elastomeric properties of the rubber. Each objective can be accomplished by the practice of the present invention, where polyorganosiloxane rubber substrate is replaced by co-homopolymerized network(s) of polyorganosiloxane/polyvinyl-based polymer(s) in a co-homopolymerization process. Polyorganosiloxane/polystyrene-based co-homopolymers are particularly preferred for use in the first stage of the graft polymers of the present invention.
None of the references disclose the in-situ co-homopolymerization of vinyl-based monomers in the presence of siloxanes in an emulsion system, as described hereinbelow. Furthermore, the present invention is also directed to the graft polymers provided by subsequent graft polymerization of vinyl aromatic-based monomers (e.g. polystyrene, styrene/acrylonitrile copolymer, styrene/divinylbenzene copolymer or styrene/acrylonitrile/divinylbenzene terpolymer) in the presence of the polyorganosiloxane/polyvinyl-based polymer product of the foregoing step.
Surprisingly, novel, extruded, molded or heat formed articles having a flatted surface can be prepared from thermoplastic resins comprising an effective amount of polyorganosiloxane/polyvinyl-based graft polymers as a flatting agent. Unexpectedly, articles comprising this flatting agent are also seen to have an increase in impact strength. None of the prior art attempts at gloss reduction suggest that the use of a polyorganosiloxane-based composition as a flatting agent would have any effect on the surface gloss properties of articles comprising such compositions.